Wireless communications systems are widely deployed to provide various types of communication such as voice, packet data, and so on. These systems may be based on code division multiple access (CDMA), time division multiple access (TDMA), frequency division multiple access (FDMA), or other multiple access techniques. For example, such systems can conform to standards such as Third-Generation Partnership Project 2 (3gpp2, or “cdma2000”), Third-Generation Partnership (3gpp, or “W-CDMA”), or Long Term Evolution (“LTE”). In the design of such communications systems, it is desirable to maximize the capacity, or the number of users the system can reliably support, given the available resources. Several factors impact the capacity of a wireless communications system, some of which are described below.
For example, in a voice communications system, a vocoder is often employed to encode a voice transmission using one of a plurality of variable encoding rates. The encoding rate may be selected based on, e.g., the amount of speech activity detected during a particular time interval. In a vocoder for a cdma2000 wireless communication system, for example, speech transmissions may be sent using full rate (FR), half rate (HR), quarter rate (QR), or eighth rate (ER) frames, with a full rate frame containing the greatest number of traffic bits, and an eighth rate frame containing the least number of traffic bits. An eighth rate frame is usually sent during periods of silence, and generally corresponds to the lowest-rate transmission that may be achieved by the voice communications system.
While an eighth rate frame represents a reduced-rate transmission in a cdma2000 system, the eighth rate frame still contains a non-zero number of traffic bits. During certain intervals, e.g., relatively long periods wherein there is no speech activity and background noise remains constant, even the eighth rate frame transmissions may unnecessarily consume a significant level of transmission power in the system. This may raise the level of interference caused to other users, thereby undesirably decreasing system capacity.
It would be desirable to provide techniques to further decrease the transmission rate of a voice communications system below what minimum-rate frame transmissions such as eighth rate frame transmissions can provide.
In another aspect of a wireless communications system, transmissions between two units often employ a degree of redundancy to guard against errors in the received signals. For example, in a forward link (FL) transmission from a base station (BS) to a mobile station (MS) in a cdma2000 wireless communications system, redundancies such as fractional-rate symbol encoding and symbol repetition may be employed. In a cdma2000 system, encoded symbols are grouped into sub-segments known as power control groups (PCG's) and transmitted over the air, with a fixed number of PCG's defining a frame.
While symbol redundancy techniques such as those employed in cdma2000 may allow accurate recovery of transmitted signals in the presence of errors, such techniques also represent a premium in the overall system transmission power when signal reception conditions are good, which may also undesirably decrease the system capacity.
It would be further desirable to provide efficient techniques to, for example, terminate transmission of a frame when it is determined that the receiver has accurately recovered the information associated with that frame, thereby saving transmission power and increasing the system capacity. It would be further desirable to provide modified power control schemes to accommodate such techniques.